As part of the manufacturing process of semiconductor devices, semiconductor wafers are increasingly being polished by CMP. The uniform removal of material from and the planarity of patterned and un-patterned wafers is critical to wafer process yield. Generally, the wafer to be polished is mounted on a substrate carrier which holds the wafer using a combination of vacuum suction or other means and, most often, a wafer backing pad to contact the rear side of the wafer. A retaining lip or ring is generally provided around the edge of the wafer to keep the wafer contained under the substrate carrier. The front side of the wafer, the side to be polished, is then contacted with an abrasive material such as an abrasive pad or abrasive strip. The abrasive pad or strip may have free abrasive fluid sprayed on it, may have abrasive particles affixed to it, or may have abrasive particles sprinkled on it.
The ideal wafer polishing process can be described by Preston's equation: EQU R=K.sub.p *P*V,
where R is the removal rate; Kp is a function of consumables (abrasive pad roughness and elasticity, surface chemistry and abrasion effects, and contact area); P is the applied pressure between the wafer and the abrasive pad; and V is the relative velocity between the wafer and the abrasive pad. As a result, the ideal CMP process should have constant cutting velocity over the entire wafer surface, constant pressure between the abrasive pad and wafer, and constant abrasive pad roughness, elasticity, area and abrasion effects. In addition, control over the temperature and pH is critical and the direction of the relative pad/wafer velocity should be randomly distributed over the entire wafer surface.
One common type of wafer polishing apparatus is the CMP model 372M made by Westech Systems Inc. A wafer is held by a substrate carrier of the model 372M. The substrate carrier rotates about the axis of the wafer. A large circular abrasive pad is rotated while contacting the rotating wafer and substrate carrier. The rotating wafer contacts the larger rotating abrasive pad in an area away from the center of the abrasive pad.
Another related apparatus is a polishing machine for polishing semiconductor wafers containing magnetic read-write heads, disclosed in U.S. Pat. No. 5,335,453 to Baldy et al. With this machine, a semiconductor wafer is held by a substrate carrier which is moved in a circular translatory motion by an eccentric arm. The wafer is polished by contacting an abrasive strip which is advanced in one direction. The relative motion between the wafer and the abrasive strip is a combination of the circular motion of the wafer and the linear motion of the advancing abrasive strip. Connected to the eccentric arm is a support head that includes a rigid part and a "flexible disk" made from a "flexible material" having a "certain thickness". The wafer 44 to be polished is described as being "partly embedded in the disk 142 during polishing by the effect of the force exerted on the support head".
The gimbal point of a CMP substrate carrier is a critical element of the polishing process. The substrate carrier must align itself to the polish surface precisely to insure uniform, planar polishing results. Many CMP substrate carriers currently available yield wafers having anomalies in planarity. The vertical height of the pivot point above the polishing surface is also important, since the greater the height, the larger the moment that is induced about the pivot point during polishing. Two pervasive problems that exist in most CMP wafer polishing apparatuses are underpolishing of the center of the wafer, and the inability to adjust the control of wafer edge exclusion as process variables change.
For example, substrate carriers used on many available CMP machines experience a phenomenon known in the art as "nose diving". During polishing, the head reacts to the polishing forces in a manner that creates a sizable moment, which is directly influenced by the height of the gimbal point, mentioned above. This moment causes a pressure differential along the direction of motion of the head. The result of the pressure differential is the formation of a standing wave of the chemical slurry that interfaces the wafer and the abrasive surface. This causes the edge of the wafer which is at the leading edge of the substrate carrier, to become polished faster and to a greater degree than the center of the wafer.
The removal of material on the wafer is related to the chemical action of the slurry. As slurry is inducted between the wafer and the abrasive pad and reacts, the chemicals responsible for removal of the wafer material gradually become exhausted. Thus, the removal of wafer material further from the leading edge of the substrate carrier (i.e., the center of the wafer) experiences a diminished rate of chemical removal when compared with the chemical action at the leading edge of the substrate carrier (i.e., the edge of the wafer), due to the diminished activity of the chemicals in the slurry when it reaches the center of the wafer. This phenomenon is sometimes referred to as "slurry starvation".
Apart from attempts to reshape the crown of the substrate carrier, other attempts have been made to improve the aforementioned problem concerning "nose diving". In a prior art substrate carrier that gimbals through a single bearing at the top of the substrate carrier, sizable moments are generated because the effective gimbal point of the substrate carrier exists at a significant, non-zero distance from the surface of the polishing pad. Thus, the frictional forces, acting at the surface of the polishing pad, act through this distance to create the undesirable moments.
U.S. Pat. No. 5,377,451 to Leoni et al. describes a wafer carrier that "projects" the effective gimbal point down to the surface of the polishing pad, thereby eliminating the moment arm through which the frictional forces create the undesirable "nose diving". Leoni et al. produce this effect by instituting a conical bearing assembly which allows the projection of a "universal pivot point" to a point that is located at or near the surface of the polishing surface. The solution proposed by Leoni et al., however, requires the use of a number of bearings in the assembly in order to effect this projection, thereby increasing the cost of the wafer carrier. Additionally, there is still a moment produced because of the actual contact points at the bearings. There is also a substantial risk that, due to inexact manufacturing, the projected pivot point will not lie exactly on the contact surface of the carrier, which will also introduce moments.
FIG. 17 shows a prior art carrier design 900 which transfers the polishing load from a bellows 910 to a guided shaft 920 into a gimbal 930 (shown in phantom to illustrate the gimbal point 933 and outward into a carrier plate 940. If the gimbal mechanism is not free, stiction will prevent the gimbal 930 from its intended free and smooth movement and the guided shaft 920 will begin to over-constrain the system during polishing.
Additionally, it is not uncommon for loads in this type of a system to become excessive enough to cause plastic deformation of the gimbal. Because of the offset rotation points of the gimbal 930 and the ring flexure 950, the dynamics of such a carrier assembly can become unstable during a high friction polishing operation.
A semiconductor wafer polishing apparatus by Banks in U.S. Pat. No. 4,373,991, uses a plurality of channels 27 to inject pressurized water, preferably slightly greater than 15 psi, between a plate and a wafer to allow free floating of the wafer. However, the carrier of Banks uses a conventional gimbal arrangement and therefore experiences the moment induced anomalies such as nose-diving and crowning, as discussed above.
Another phenomenon which generates anomalies in the edge areas of a substrate that is polished by conventional techniques is due to limitations inherent in a carrier that employs a deformable/conformable crown or plate. For example Applied Materials European Patent Application No. EP 0 774 323 A2 discloses a carrier head having a lower planar surface 9104 and a bow chamber 9102 which is capable of being pressurized so as to bow out the surface 9104, or reduced in pressure to bow in the surface 9104. A bellows cavity 1192 is pressurizable to bias the entire carrier plate 1164, including the surface 9104 toward the polishing surface for loading the substrate to be polished.
FIG. 18 illustrates a problem inherent in a prior art carrier 1100 having a deformable plate 1110. Upon deformation of the plate 1110 by application of pressure thereto, either through increasing the pressure within chamber 1130 or by other means, the deflection of the plate 110 is greater toward the center of the plate than at the edge areas 1120 (as shown in phantom in FIG. 16). This is true even if greater flexibility is afforded at the edge areas through living hinges or other mechanisms to extend the flexibility outward, since the very edge defines a boundary of fixed points that do not deflect.
The plate 1110 deflects according to the typical bending formula (as shown in phantom in FIG. 16) which results in a relative underpolishing of the edges of the wafer.
U.S. Pat. No. 5,635,083 to Breivogel et al., discloses a method and apparatus for chemical mechanical polishing of a substrate having a wafer carrier attached to a steel rotatable drive shaft which is hollow to allow pneumatic pressure to be conveyed into a chamber created above the backside of a wafer to be polished and below the base of the carrier. A wear resistant retaining ring extends from the base of the carrier and surrounds and is in contact with the wafer to be polished. A resilient lip seal is attached just inside the retaining ring and seals with the backside of the wafer to form the chamber together with the base of the carrier. Not only does this arrangement restrict wafer precession because of the seal contact, but there is also always a risk of not forming an adequate seal due to contamination between the seal and the backside of the wafer, by slurry or other contaminants.
An apparatus described in JP 9-225821 to Ebara Corp. includes first, second and third pressure chambers within a top ring that is used to polish a semiconductor wafer. An elastic mat is provided between the top ring and the semiconductor wafer to be polished. The elastic mat and the top ring each have multiple jets which align to connect with the pressure chambers. Three concentrically defined pressure zones are defined on the mat, through which controlled pressures can be applied to the wafer to control the conformation of the pressure profile between the elastic mat and the semiconductor wafer.